1. Field of the Invention
The present invention relates to a digital signal transfer device that transfers a digital input signal by means of an isolating transfer system.
2. Description of the Related Art
Conventionally, digital signal transfer circuits that transfer a digital input signal from the input side to the output side in a condition in which the input side and output side are electrically isolated are employed in various types of circuit such as for example gate control circuits of inverters.
In such digital signal transfer circuits, a photocoupler is employed as means for achieving isolating transfer in which signal transfer is performed with the input side and output side electrically isolated. Examples are to be found in Toshiba Ltd Compact IPM application notes, Internet search <URL:http://www.semicon.Toshiba.co.jp/prd/tr/doc/db_bdjoo37a.h tml> and Toshiba Ltd photocoupler/photorelay product guide, Internet search <URL:http://www.semicon.Toshiba.co.jp/prd/opto/doc/catalog—143 62clap.html>.
In a digital signal transfer circuit in which a photocoupler is employed as isolating transfer means, when the circuit is employed for a long time, transfer of digital signals frequently becomes impossible due to deterioration of the photocoupler. Also, regarding the environment of use of the circuit, a photocoupler cannot be used in a high temperature environment of 100° C. or more for reasons to do with the photocoupler performance. Also, a photocoupler is unsuitable in particular for digital signal transfer circuits that perform high-speed signal transfer, because of signal delay of about a few hundred n sec (nano seconds) to a few μ sec (micro seconds) between input and output.
An object of the present invention is therefore to provide a novel digital signal transfer device of the isolating transfer type having superior performance wherein the problems of a photocoupler as isolating transfer means can be eliminated.